The objectives of this project are the development of procedures for classification of spotted fever and typhus group rickettsiae and the determination of the nature and biological properties of rickettsial antigens and constituents. From studies utilizing crossed and tandem crossed immunoelectrophoresis, a scheme for the antigenic composition of Rickettsia rickettsii, R. conorii, and R. australis was devised. Localization of an antigen possibly of significance in immunity was accomplished by immunoelectron microscopy. A group antigen was also identified. Antiserum to the group antigen reacted in the micro-immunofluorescence test with representatives of all known serotypes of the spotted fever group. Specificity and persistence of antibodies in sera from Rocky Mountain spotted fever patients were determined by crossed immunoelectrophoresis with an intermediate gel. Two new rickettsial isolates, one representing a widely-distributed serotype, were assigned to the typhus group on the basis of their DNA composition.